Dish Solar Concentrator Design

Over the next few months I will describe how I design point focus solar collector that will provide all the hot water and half the heat for my home. I will document how I manufacture the parts, assemble them, erect the structure, mount and align the mirror assemblies, connect the drives and plumb the receiver. The illustration below shows a computer model of an early version that has evolved over the past year.

Mirror Assemblies: Size, Quantity and Configuration

I will start by describing how I make mirror assemblies. They make up the concentrator and focus sunlight on a receiver designed to harness concentrated sunlight efficiently. Later I will cover the design of girders and trusses that support these components, the foundations required, and the gimbal that carries the concentrator and receiver. These all rotate together to follow the daily motion of the sun as well as allow all to follow the sun as it goes higher and lower in the sky through the seasons.

Computer Model of a Solar Dish with Mirror Assemblies and Receiver

A solar dish collector uses a concentrator to reflect sunlight into a receiver that transforms radiant energy into hot water or steam. The area of the concentrator may be 500 to 1,000 times larger than the opening of the receiver to minimize energy losses.  The receiver can be mounted close to the reflector, far away, or somewhere in between. Dividing the focal length, the distance from the mirrors to the receiver, by the concentrator diameter describes this (f/d) relationship. I work with an f/d ratio of 0.8, knowing that this works well with very high performance cavity receivers. Shorter focal lengths, say 0.6, require shallow cavities that lose more energy than deeper ones because hot surfaces inside lose more heat to wind and radiation. A longer focal length, say 1.0, where the receiver aperture is the same distance from the mirrors as the concentrator is wide, extends the receiver farther out from the main structure and requires more material and higher quality mirror assemblies.

Illustration Showing Various Focal Length to Diameter Ratios

Many point focus solar collectors have individual mirrors mounted on a structure to reflect images of the sun on a target. Attaching and aiming each one takes time and these individual mirror facets are difficult to clean. We made early solar collectors in the 1970s this way. We progressed to mechanically curved mirror assemblies that placed eight one-foot square mirrors end to end and were handled as a larger unit. Correct curves insure mirrors reflect sunlight to a spot. Mirrors are then continuous and easy to squeegee clean. Accurately curving aluminum extrusions is straightforward and simply attaching mirrors to them can establish a parabolic reflective surface that intensifies sunlight 30 to 60 times without having to adjust any mirrors. Attaching a few dozen of these assemblies to a structure and aligning them so they reflect sunlight to a single point creates the point focus concentrator. Permanently mounting mirrors in arrays minimizes labor when erecting a collector. An ideal: assemble and erect a solar collector in a single day. Accurately aligning mirror assemblies is easier and quicker than individually mounting nine to 16 times as many smaller mirror facets.

A specific design begins by choosing an appropriate size mirror assembly and then settling on a suitable array of these assemblies to make up the concentrator. Mirror facets come in many sizes and what I have on hand are 12”x12” and 12” x 36” mirrors that readily fit into square panels that are either three or four feet square or rectangles that are three by four foot. One person can readily handle these mirror assemblies. A 3’ x 3’ weighs 10 pounds and the larger ones weigh less than 20 pounds.

The table below shows a few ways that mirror assemblies can be arranged in a concentrator. A roughly circular arrangement gives each mirror a good view of the inside of the receiver. The farther away from the optical axis (a line through the centers of the concentrator and receiver) a mirror is, the poorer view it has of the inside of a cavity receiver, making it less effective than those closer to the center.

Mirror Assembly Configurations with Table of Mirror Area & Quantity

At this point I’m leaning toward configuration “C” that has 16 four-foot square mirror assemblies. In an hour in bright sun these would reflect 24 square meters of sunlight, 20 or more kilowatts, into a receiver that would displace burning five pounds of fuel oil and releasing over 11 pounds of carbon dioxide when I burned oil for heat and hot water. Over a year it should halve how much wood I have to gather and work into pieces for the stove: maybe by three cords, or six tons. I should be able to manufacture 16 mirror assemblies in a day or two and to fix and align them on the concentrator structure in one night so they hit a target.

It is easier to work in the dark using an LED flashlight shining at a mirror assembly, MA, from the center of a target at twice the focal length. Tighten the bolts at the four corners when it reflects the image to the target center. One could use sunlight during the day to do this but after aligning a few mirror assemblies the target becomes so bright it becomes impossible to see the reflection of a single MA. One could cover each fixed MA but this is cumbersome. Also, using the sun during the day works well around noon, when the concentrator faces up and is level, but because the structure must be tracking the sun for the process to work, climbing around the tilted moving structure in the morning or late afternoon becomes more difficult. And sunny days without clouds often don’t occur when you want them. Every night is dark, though, and working close to the ground with a stationary structure looks easier so we will be exploring this new MA mounting and alignment technique for this project.

Choosing a Solar Collector

 Peak oil, climate change and sustainability require developing energy resources that are freely available, affordable and don’t alter the atmosphere or oceans. Why not start providing energy for our homes and public buildings because they don’t move and they consume a third of our fossil fuels? Wind turbines feed power into the grid but many homes and farms do not have reliable wind for direct power, hot water and comfort. Sun shines everywhere and solar collectors can augment many living situations. There are many kinds of solar collectors: flat plate types that simply utilize the sunlight that shines on them, those that intensify sunlight from a few times and up to 50 times, and those that concentrate solar energy a few hundred to over 1,000 times. Which is most useful?

Earth is in the sun’s Goldilocks zone: not too hot, and too cold. Natural sunlight is just right. We wouldn’t be here if the sun didn’t support plants and animals. But to gather enough sunlight to do the useful work we’ve come to expect requires utilizing energy from large areas. Economics favor using inexpensive mirrors to intensify sunlight on a small area that will pay for high-tech approaches (like high temperature materials and insulation) to harvest most of the available energy. Flat plate solar collectors, both photovoltaic and thermal, use materials that would be too expensive without government subsidies and perform poorly (PV delivers <15% and thermal even less when it’s cold). Passive solar designs use windows to contribute heat for buildings but today’s active systems cost too much.

Prototype Dish Solar Collector: Work Begins in 2015 on One That Has About Four Times More Mirror Area

Parabolic trough solar collectors that intensify sunlight from 20 to 50 times are popular for generating power and typically use glass envelopes to surround the metal tube that contains the working fluid inside with the volume between evacuated like a Thermos bottle. To allow the metal and glass to expand at different rates without breaking, they need a flexible metal bellows to join the two materials.  These receiver assemblies are quite expensive and when they lose vacuum they must be drained, cut out and a new one welded in place. Again, without subsidies, they are too expensive.

Concentrating photovoltaic collectors, parabolic dishes and solar power towers intensify sunlight from 300 to a few thousand times. This allows receivers to work well without vacuum enclosures because the high intensity energy can be efficiently captured without a window or vacuum enclosure (typically capturing above 90% even when it's below freezing). Economics favor higher concentrations because inexpensive optical elements direct large amounts of sunlight into tiny receivers. Using 450 mirrors, a square foot each at a dollar, to intensify sunlight 900 times requires a small, well-insulated trash pail size receiver with a 9-inch diameter opening to collect 34 kilowatts in bright sunlight. Each hour this collector would deliver the same amount of steam as a fossil fueled boiler burning a gallon of fuel oil or 1.5 gallons of propane. The same concentrator with a CHP (combined heat and power) receiver would deliver 10 kilowatts of power and 20 of heat.

Taking Apart the Engineering Prototype Solar Dish

The few dish solar collectors available today are complicated and require experts and cranes to install them. This year I hope to demonstrate a new approach to building solar dishes that enables anyone to make the parts in their home shop, put them together, accurately align mirrors and let a simple open-source controller run it for 30 years, custom programming and setup not required. I hope that the availability of inexpensive and fun to make solar equipment will enable others to develop receivers and applications that enable very high performance renewable energy systems to displace major amounts of fossil fuels.